In larger PV power plants with DC/AC-converters (inverters), the plant is typically connected to the power grid through a dedicated isolation transformer, which connects the relatively low voltage PV generator system to the medium voltage power grid. One reason for this is that the PV modules, which convert the solar energy into electrical energy, typically must have a defined potential with respect to ground. This is typically achieved by grounding all or some of the PV modules.
Grounding is normally done in order to comply with local regulations, to facilitate the detection of isolation faults and/or to avoid corrosion and/or yield reduction of the PV modules.
Detection of isolation faults may be difficult in larger systems due to the rather high leakage currents from the PV modules, especially in wet conditions. By grounding the system, leakage currents can be monitored.
Some types of PV module, notably thin film modules incorporating a TCO (transparent conductive oxide) layer are prone to irreparable damage, and consequent substantial power losses, resulting from the reaction of glass-sodium with moisture. To avoid accelerated degradation of such PV modules, it is normally required to ground the negative terminal of the PV strings, i.e. avoid that any active part of the PV modules have a negative potential with respect to the ground potential. The degradation of the PV modules depends on the potential difference between the active parts of the module and the ground. Depending on the module construction, grounded parts may be in very close distance from the active parts—accelerating the degradation.
With some other types of PV module, notably those where the terminals are all located on one side of the module—known as ‘back contact modules’—a reduction of module efficiency has been observed during operation. This appears to be due to a build up of static charge on the surface of the cell and can be counteracted by maintaining the cell below the ground potential. Thus, some back-contact PV modules require that the positive terminal is grounded in order to avoid yield losses, i.e. their terminals must have non-positive potentials.
In larger PV systems comprising several strings of PV modules and several inverters, grounding of more than one PV module may cause currents to run through the ground (ground loops). Ground loops may cause problems with controlling the power plant, increase the risk and/or the rate of corrosion and also increase problems relating to electromagnetic interference (EMI). In order to avoid ground loops, transformer-based inverters may be used, so that the DC and the AC sides of the inverters are separated galvanically. Such inverters are however, relatively heavy and expensive, and there is a demand for PV power plants, which may utilise transformer-less inverters and still ensure defined potentials with respect to ground at the PV modules. The use of transformer-less inverters in large PV power plants does however, require that, if ground loops shall be avoided, the PV generator system be configured as a network with an earthing system where the AC side of the inverters has no connection to ground at all. This is known as an ‘IT’ earthing system and is described in, for example, IEC (International Electrotechnical Commission) International Standard 60364-1—Electrical Installation in Buildings. This means, in practice, that the AC side of the system must be floating with respect to ground and can therefore not be grounded.
FIG. 1 illustrates a typical prior art power plant 22 and comprises a single PV generator 23, comprising a PV string 3 and a transformerless (non galvanically isolated) inverter 24. The inverter 24 has a DC input 18 and a three-phase AC output 19. The PV string 3 comprises three PV modules 5 connected in series and arranged so that they will be exposed to sunlight. Each PV module 5 comprises a number of PV cells (not shown) connected as already known in the art so that they generate a single DC power output at the terminals 6 of the PV module 5. The PV string 3 is electrically connected to the DC input 18 of the inverter 4 through a positive connection 7 and a negative connection 8. The AC outputs 19 are connected electrically in parallel to a power grid 9 comprising three power lines and a neutral line. The neutral line is connected to ground via a ground connection 15.
Here the system is configured as a network with an earthing system where the AC side of the inverter has a connection to ground, and the network also includes a ground connection. This is known as a ‘TN’ earthing system and is described in, for example, IEC 60364-1.
When operating, the voltages appearing at the positive input 7 and negative input 8 of the inverter 24 are represented in FIG. 2. In the graph the axis 25 represents the voltage with respect to ground, and it will be seen that the voltage at the positive input 7 (represented by the line 27) is above ground potential whilst that at the negative the negative input 8 (represented by the line 28) is below ground potential. These voltages, as well as the potential between them—the voltage across the PV string 3 (represented by the range 26)—are controlled by the characteristics of the inverter 24, the irradiation of the PV string 3, the type of solar cells used in each PV module 5 as well as other factors. Since the grounding of either side of the inverter DC inputs 18 is not possible in this design of power plant, such a power plant 22 will be subject to a decrease in efficiency, and the PV string 3 liable to damage, resulting from the problems discussed above.
FIG. 3 illustrates another prior art power plant 29. Here the difference from the power plant 22 of FIG. 1 is that the PV generator 31 comprises a transformer-based (galvanically isolated) inverter 30, that is to say there is galvanic isolation between the DC input 18 and AC output 19 of the inverter. This allows the grounding of the negative input 8 of the inverter 30 to be made using a ground connection 32. FIG. 4 illustrates the voltages appearing at the DC inputs 18 of the inverter 30 in a similar manner to FIG. 2. In can be seen that the whole PV string 3 is held at a positive potential relative to ground. Such a configuration is suitable for avoiding the problems with thin film modules discussed above. If, alternatively, the positive input 7 of the inverter 30 was grounded instead of the negative input 8, then a configuration suitable for back contact type modules would be realised.
In this type of power plant, whilst it is possible to control the voltages appearing at the inputs 18, and so minimise the decrease in efficiency and damage resulting from the problems discussed above, this advantage comes only at the cost of using a transformer-based inverter (30). Such an inverter design is more expensive to produce, heavier and is less efficient in operation and so the use of such an inverter is clearly a disadvantage. A further disadvantage of this type of power plant is the requirement that a ground connection 32 needs to be physically connected to the positive input 7 or the negative input 8 of the inverter 30. This requires additional hardware and labour to attach. In addition, changing the type of PV modules at a later date may involve the physical disconnection and/or reconnection of a ground connection 32, a procedure which is labour intensive and therefore a disadvantage.
FIG. 5 illustrates yet another prior art power plant 33. Here the difference from the power plant 22 of FIG. 1 is that the AC outputs 19 are connected electrically in parallel to a power grid 9 through a three-phase AC connection 17 and a three-phase isolation transformer 10 having a primary side 11, a secondary side 12 and a neutral terminal 13 on the primary side. Such a transformer is often used in high capacity power plants, where multiple inverters are coupled in parallel, and is described in more detail below. Such a configuration allows the potential of the inverter inputs 18 to be independent of the potential of the network 9 without the need for using a costly transformer-based inverter 31. The grounding of the negative input 8 of the inverter 24 can be made using a ground connection 32. FIG. 4 illustrates the voltages appearing at the DC inputs 18 of the inverter 24 in a similar manner to FIG. 2. In can be seen that the whole PV string 3 is held at a positive potential relative to ground. Such a configuration is suitable for avoiding the problems with thin film modules discussed above. If, alternatively, the positive input 7 of the inverter 24 was grounded instead of the negative input 8, then a configuration suitable for back contact type modules would be realised.
A disadvantage of this type of power plant is the requirement that a ground connection 32 needs to be physically connected to the positive input 7 or the negative input 8 of the inverter 24. This requires additional hardware and labour to attach. In addition, changing the type of PV modules at a later date may involve the physical disconnection and/or reconnection of a ground connection 32, a procedure which is labour intensive and therefore a disadvantage. In addition, if two or more PV generators 23 are connected in parallel, as illustrated in FIG. 6, and one input of each inverter 23 is earthed as described above, problems will arise if the characteristics or irradiation of each PV string 3 are not identical. This may cause unwanted voltages and consequent ground loop currents.
In all the prior art power plants illustrated above it can be seen that whilst the potential of one or more PV strings with respect to ground is an important parameter for running a power plant in an efficient manner and one which does not cause damage to the PV strings, this often only achieved by the use of expensive hardware or the labour intensive fitting of additional hardware.